Apparatus and method for electrical characterization by selecting and adjusting the light for a target depth of a semiconductor

ABSTRACT

The present disclosure provides methods and apparatus that enable characterization of an electrical property of a semiconductor specimen, e.g., dopant concentration of a near-surface region of the specimen. In exemplary method, a target depth for measurement is selected. This thickness may, for example, correspond to a nominal production thickness of a thin active device region of the specimen. A light is adjusted to an intensity selected to characterize a target region of the specimen having a thickness no greater than the target depth and a surface of the specimen is illuminated with the light. An AC voltage signal induced in the specimen by the light is measured and this AC voltage may be used to quantify an aspect of the electrical property, e.g., to determine dopant concentration, of the target region.

REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 60/433,382, filed 13 Dec. 2002 and entitled “ApparatusAnd Method For Electrical Characterization Of Semiconductor Film AndLayers,” the entirety of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to semiconductor manufacture.Aspects of the invention have particular utility in connection withcharacterization of semiconductors, e.g., determining electricalcharacteristics of a near-surface region of a semiconductor.

BIBLIOGRAPHY

A number of patents discuss the use of surface photovoltage (SPV)measurements to measure or characterize properties of semiconductors.These patents include:

-   -   U.S. Pat. No. 4,544,887 (issued 1 Oct. 1985 to E. Kamieniecki)        discloses a method of measuring the photo-induced voltage at the        surface of a specimen of semiconductor material;    -   U.S. Pat. No. 4,827,212 (issued 2 May 1989 to E. Kamieniecki)        discloses a method and apparatus for characterizing a        semiconductor using the surface photovoltage effect;    -   U.S. Pat. No. 4,891,584 (issued 2 Jan. 1990 to E.        Kamieniecki, W. C. Goldfarb, and M. Wollowitz) discloses an        apparatus for making AC surface photovoltage measurements of a        specimen of semiconductor material under DC bias voltage        conditions;    -   U.S. Pat. No. 5,087,876 (issued 11 Feb. 1992 to L. Reiss and E.        Kamieniecki) discloses an apparatus for making AC surface        photovoltage measurements of a specimen of semiconductor        material under variable DC bias voltage conditions;    -   U.S. Pat. No. 5,091,691 (issued 25 Feb. 1992 to E.        Kamieniecki, W. C. Goldfarb, and M. Wollowitz) discloses an        apparatus for making AC surface photovoltage measurements of a        specimen of semiconductor material under DC bias voltage        conditions;    -   U.S. Pat. No. 5,661,408 (issued 26 Aug. 1997 to E. Kamieniecki        and J. Ruzyllo) discloses an apparatus and method for the        real-time, in-line testing of semiconductor wafers during the        manufacturing process;    -   U.S. Pat. No. 6,069,017 (issued 30 May 2000 to E. Kamieniecki        and J. Ruzyllo) discloses an apparatus and method for the        real-time, in-line testing of semiconductor wafers during the        manufacturing process;    -   U.S. Pat. No. 6,315,574 (issued 13 Nov. 2001 to E. Kamieniecki        and J. Ruzyllo) discloses an apparatus and method for the        real-time, in-line testing of semiconductor wafers during the        manufacturing process;    -   U.S. Pat. No. 6,326,220 B1 (issued 4 Dec. 2001 to W-W. Chen, Y-L        Hwang and Y-C Yang) discloses a method for determining        near-surface doping concentration by utilizing surface        photovoltage; and    -   U.S. Patent Application No. 2002/0006740 (published 17 Jan. 2002        to E. Kamieniecki and J. Ruzyllo) discloses an apparatus and        method for the real-time, in-line testing of semiconductor        wafers during the manufacturing process. Other known        publications relating to surface photovoltage (SPV) measurements        of semiconductors include:    -   Dieter K. Schroder, “Semiconductor Material and Device        Characterization,” John Wiley & Sons, Inc., New York 1990,        Chapter 2;    -   E. Kamieniecki, “In-Line Process Monitoring in Advanced IC        Manufacturing,” ECS-ALTECH 99, and “Analytical and Diagnostic        Techniques for Semiconductor Materials, Devices, and Processes,”        ECS 1999, ECS Proceedings Vol. 99-16; SPIE Vol. 3895, pp.        259-270;    -   R. S. Nakhmanson, “Frequency Dependence of the Photo-EMF of        Strongly Inverted Ge and Si MIS Structures-I. Theory,” Solid        State Electronics, Vol. 18, 1975, pp. 617-626;    -   R. S. Nakhmanson, Z. Sh. Ovsyuk, and L. K. Popov, “Frequency        Dependence of Photo-EMF of Strongly Inverted Ge and Si MIS        Structures-II. Experiments,” Solid State Electronics, Vol. 18,        1975, pp. 627-634;    -   R. L. Streever, J. J. Winter, and F. Rothwarf, “Photovoltage        Characterization of MOS Capacitors,” Proc. Int. Symp. Silicon        Materials Sci. & Tech., Philadelphia, May 1977, (Electrochem.        Soc. Princeton, 1977) pp. 393-400;    -   A. Sher, Y. H. Tsuo, J. A. Moriarty, W. E. Miller, and R. K.        Crouch, “Si and GaAs Photocapacitive MIS Infrared Detectors,” J.        Appl. Phys., Vol. 51, No. 4, April 1980, pp. 2137-2148;    -   Chusuke Munakata, Mitsuo Nanba and Sunao Matsubara,        “Non-Destructive Method of Observing Inhomogeneities in p-n        Junctions with a Chopped Photon Beam,” Jpn. J. Appl. Phys., Vol.        20, No. 2, February 1981, pp. L137-L140;    -   E. Kamieniecki, “Determination of Surface Space Charge Using a        Light Probe,” J. Vac. Sci. Technol., Vol. 20, No. 3, March 1982,        pp. 811-814;    -   E. Kamieniecki and G. Parsons, “Characterization of        Semiconductor-Electrolyte System by Surface Photovoltage        Measured Capacitance,” 164th Meeting of the Electrochemical        Society, Washington, D.C., October 1983, The Electrochemical        Society, Extended Abstracts, Vol. 83-2, p. 561;    -   E. Kamieniecki, “Surface Photovoltage Measured Capacitance:        Application To Semiconductor/Electrolyte System,” J. Appl.        Phys., Vol. 54, No. 11, November 1983, pp. 6481-6487;    -   O. Engstrom and A. Carlsson, “Scanned Light Pulse Technique for        the Investigation of Insulator-semiconductor Interfaces,” J.        Appl. Phys., Vol. 54, No. 9, September 1983, pp. 5245-5251;    -   E. Thorngren and O. Engstrom, “An Apparatus for the        Determination of Ion Drift in MIS Structures,” J. Phys. E: Sci.        Instrum., Vol. 17, 1984, pp. 1114-1116; Chusuke Munakata,        Shigeru Nishimatsu, Noriaki Honma and Kunihiro Yagi, “AC Surface        Photovoltages in Strongly-Inverted Oxidized p-Type Silicon        Wafers,” Jpn. J. Appl. Phys., Vol. 23, No. 11, November 1984,        pp. 1451-1461;    -   Chusuke Munakata and Shigeru Nishimatsu, “Analysis of AC Surface        Photovoltages in a Depleted Oxidized p-Type Silicon Wafer,”        Jpn. J. Appl. Phys., Vol. 25, No. 6, June 1986, pp. 807-812;    -   Hiromichi Shimizu, Kanji Kinameri, Noriaki Honma, and Chusuke        Munakata, “Determination of Surface Charge and Interface Trap        Densities in Naturally Oxidized n-Type Si Wafers Using AC        Surface Photovoltages,” Jpn. J. Appl. Phys., Vol. 26, No. 2,        February 1987, pp. 226-230;    -   E. Kamieniecki, “Surface Charge Analysis: A New Method to        Characterize Semiconductor/Insulator Structures—Application to        Silicon/Oxide System,” Proceedings of the 1st International        Symposium on Cleaning Technology in Semiconductor Device        Manufacturing, Hollywood, Fla., October 1989, Vol. 90-9, pp.        273-279;    -   E. Kamieniecki and G. Foggiato, “Analysis and Control of        Electrically Active Contaminants by Surface Charge Analysis,”        Handbook of Semiconductor Wafer Cleaning Technology, Part IV-11,        Ed. W. Kern, Noyes Publications (1993); and D. Marinskiy, J.        Lagowski, M. Wilson, L. Jastrzebski, R. Santiesteban, and Kim        Elshot, Small Signal ac-Surface Photovoltage Technique for        Non-Contact Monitoring of Near Surface Doping for        IC-processing,” In Process Control and Diagnostics, Michael L.        Miller, Kaihan A. Ashtiani, Editors, Proceedings of SPIE Vol.        4182 (2000), pp. 72-77.

BACKGROUND

The performance of an integrated circuit depends in part on theelectrical properties of materials defining the circuit. One importantregion for many integrated circuits is the “active device region,” whichis located close to the surface, e.g., of a semiconductor wafer. Withincreasing complexity of integrated circuits, this active device regionmay be formed by depositing a high quality film, typically an epitaxiallayer, of the same material as the substrate (e.g., Si) or a compatiblematerial such as SiGe (silicon-germanium). The top layer could also be astrain layer (strain silicon) or a film formed on an insulator (e.g.,silicon-on-insulator or “SOI”). Such films may even be separated fromthe substrate by an air gap (e.g., “silicon-on-nothing” or “SON”). Thethickness of the active device region scales with dimensions of thedevices forming the integrated circuits. Smaller devices, which arerequired by the increasing density of integrated circuits, generallyrequire a thinner active device region and a thinner high quality filmor layer in which these devices are formed. Many advanced integratedcircuits employ films that are no more than about 400 nm thick, withsome of them having a thickness of 100 nm or less; such films aresometimes referred to herein as “thin active films.”

Achieving respectable manufacturing yields when using thin active filmsrequires reliable information regarding the electrical properties of thethin active film. Electrical properties of the near-surface region ofsemiconductor materials are conventionally measured using capacitancevs. voltage (C-V) measurements. As schematically illustrated in FIG. 1,a small AC voltage V_(AC) is applied to the semiconductor device 20,which is typified in FIG. 1 as a metal-oxide-semiconductor capacitor or“MOS-C,” using a measurement circuit 10. The measured current J_(m) maybe used in a known fashion to determine the capacitance of the surfacespace-charge region. Simultaneously applied DC voltage VDC may be variedto modify this space-charge region and determine various electricalcharacteristics, including doping concentration. However, the C-Vtechniques cannot accurately measure the near-surface resistivityprofile to less than about 2 to 3 Debye lengths from the surface (morethan 500 nm for 10 ohm-cm silicon). (Dieter K. Schroder, “SemiconductorMaterial and Device Characterization,” John Wiley & Sons, Inc., New York1990, Chapter 2). Measurements of average doping in the near-surfaceregion are possible by so-called “Maximum-Minimum MOS-C Capacitance.”This method compares a maximum capacitance and a minimum capacitance forthe tested specimen, which are achieved by applying the appropriate highelectrical DC bias. Unfortunately, if this approach is used fornon-contact measurements using an air gap, for example, the requisiteelectrical field can be high enough to cause electrical breakdown of andsurface damage to the tested specimen.

Some known alternatives to the C-V technique are based on the surfacephotovoltage (SPV) effect described in U.S. Pat. Nos. 4,544,887;4,827,212; 4,891,584; 5,087,876; 5,091,691; and 5,661,408, for example.These approaches are also extensively discussed in a number ofpublications, e.g., E. Kamieniecki, 1999; R. S. Nakhmanson, 1975; R. S.Nakhmanson at al., 1975; R. L. Streever at al., 1977; A. Sher at al.,1980; Ch. Munakata at al., 1981; E. Kamieniecki, 1982; E. Kamieniecki atal., 1983; E. Kamieniecki, 1983; O. Engstrom at al., 1983; E. Thorngrenat al., 1984; Ch. Munakata at al., 1984; Ch. Munakata at al., 1984; H.Shimizu, 1987; E. Kamieniecki, 1989; E. Kamieniecki at al., 1993; and D.Marinskiy at al., 2000 (each of which is identified in the precedingbibliography). Currently, SPV techniques for measuring near-surfacedoping concentration use the surface photovoltage induced by anintensity modulated (or “chopped”), low-intensity, short-wavelengthlight beam (shown schematically as L in FIG. 2). This light beam Lilluminates the semiconductor surface through a transparent,electrically conductive electrode 30 that is separated from the surfaceof the specimen 20 by a fraction of a millimeter. A near-surfacedepleted region D of the measured semiconductor is under depletionconditions, i.e., is depleted of majority carriers (electrons in n-typesemiconductors and holes in p-type semiconductors). In someapplications, the surface may also be strongly inverted, i.e., withminority carriers present at the surface. The depletion or depletion andinversion is achieved by forming a surface charge 25 of polaritycorresponding to the majority carriers, as illustrated schematically inFIG. 3 for a p-type semiconductor 20. This charge could be associatedwith the natural state of the surface or it could be deposited, e.g., bycorona charging. The electrical field in the semiconductor due to thesurface charge repels majority carriers, leaving the depletion region Dadjacent the surface.

The photon energy of the light beam L impinging on the semiconductor(FIG. 2) typically exceeds the energy gap of the semiconductor 20, whichgenerates excess electron-hole pairs in the near-surface region. Theelectrical field present in the near-surface region causes excessminority carriers 26 (electrons in p-type semiconductors) to accumulateat the surface. The low-intensity, intensity modulated illuminationplays a role analogous to that of AC voltage in a C-V technique. Bykeeping light intensity low enough, the charge from the accumulatedcarriers 26 caused by the light is much smaller than the pre-existingsurface charge 25. As a result, the width of the depletion region D isnot materially affected. An alternating current surface photovoltage(AC-SPV) generated with such low-intensity light is proportional to thedark (equilibrium) width of the depletion layer D (which, in turn, isproportional to the reciprocal of the space charge capacitance). Thissignal can be used as an equivalent of electrically measuredcapacitance. (See, e.g., U.S. Pat. No. 4,544,887.) If a near-surfaceregion is depleted but the surface is not inverted, surface traps willsubstantially reduce accuracy of doping concentration measurements. Withknowledge of the maximum depletion layer width (inverted conditions)under thermal equilibrium, typically determined empirically, one maydetermine the average doping concentration in the depletion region,similarly to the “Maximum-Minimum MOS-C Capacitance” method.

As noted above, thin active films in state-of-the-art semiconductordevices may have a thickness of 100 nm or less. Unfortunately, themaximum width of the depletion layer D₀ under thermal equilibriumconditions may substantially exceed the thickness of a region ofinterest. For example, the width of the depletion region D₀ may exceed,by an order of magnitude or more, the thickness of an active deviceregion formed, e.g., by ion implantation in a near-surface region ofsilicon, in a thin active film of silicon epitaxial layer, in SiGe, instrain silicon, or in silicon-on-insulator (SOI).

Hence, conventional metrology techniques cannot effectively evaluatethin active films, which are a central part of next-generationintegrated circuits. In the absence of a reliable, flexible metrologysystem, manufacturers likely will encounter poor yields of functioningdevices with thin active films, driving up costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a conventional system for determiningsurface characteristics using capacitance-voltage measurements.

FIG. 2 schematically illustrates a conventional system used to measuresurface characteristics employing surface photovoltage effects.

FIG. 3 schematically illustrates a semiconductor having a near-surfacedepleted region.

FIG. 4 schematically illustrates a semiconductor characterization systemin accordance with one embodiment of the invention.

FIG. 5 schematically illustrates a semiconductor characterization systemin accordance with a further embodiment of the invention employing asingle light emitter.

FIG. 6A graphically illustrates time dependence of a first lightintensity generated by the semiconductor characterization system of FIG.5.

FIG. 6B graphically illustrates time dependence of a second lightintensity generated by the semiconductor characterization system of FIG.5.

FIG. 7 is a more detailed schematic illustration of a semiconductorcharacterization system that employs two light emitters in accordancewith an alternative embodiment of the invention.

FIG. 8 schematically illustrates selected portions of a semiconductorcharacterization system in accordance with still another embodiment ofthe invention.

FIG. 9 is a schematic cross-sectional view taken along line 9-9 of FIG.8.

FIG. 10 schematically illustrates idealized illumination of a targetarea of the semiconductor specimen using the characterization system ofFIGS. 8 and 9.

FIG. 11 schematically illustrates the intensity of the illuminationshown in FIG. 10.

FIG. 12 is a schematic band diagram illustrating aspects of the behaviorof an idealized semiconductor specimen in response to the illuminationshown of FIGS. 10 and 11.

FIG. 13 schematically illustrates surface potential of the idealizedsemiconductor specimen of FIG. 12 in response to the illumination shownof FIGS. 10 and 11.

FIG. 14 graphically depicts a dopant concentration profile generated inaccordance with an embodiment of the invention.

FIG. 15 graphically depicts the relationship between optical generationrate and the imaginary component of a normalized AC-SPV signal forn-type silicone with a doping concentration of about 10¹⁶ cm⁻³.

FIG. 16 graphically depicts the dependence of surface potential barrierheight on the optical generation rate of excess carriers.

FIG. 17 graphically depicts the dependence of the measured AC-SPV signalon the optical generation rate for two calibration specimens, one havinga doping concentration of about 10¹⁶ cm⁻³ and the other having a dopingconcentration of about 2×10¹⁴ cm⁻³.

FIG. 18 graphically depicts dependence of surface potential barrierheight on the optical generation rate for the specimen of FIG. 17.

DETAILED DESCRIPTION

Overview

Various embodiments of the present invention provide apparatus andmethods for determining select electrical characteristics ofsemiconductors. The following text discusses aspects of the invention inconnection with FIGS. 1-18 to provide a thorough understanding ofparticular embodiments. A person skilled in the art will understand,however, that the invention may have additional embodiments, or that theinvention may be practiced without several of the details of theembodiments shown in FIGS. 1-18.

One embodiment of the invention provides a method of characterizing anaspect of a semiconductor specimen. In this method, a target depthcorrelated to a depth of interest for the specimen is selected. A lightintensity of a light that has a first component and an intensitymodulated second component may be selected to characterize a targetregion of the specimen having a thickness no greater than the targetdepth. The light may be adjusted to the selected light intensity byvarying at least the first component of the light and a target area of asurface of the specimen may be illuminated with the adjusted light. AnAC voltage signal (which is associated with an aspect of the targetregion of the specimen) induced in the specimen by the light may bemeasured.

A method of characterizing an aspect of a semiconductor specimen inaccordance with another embodiment of the invention may include charginga surface of the specimen to produce an inverted surface and a firstdepletion layer in a near-surface region. The first depletion layer hasa width corresponding to a dark equilibrium value. The surface of thespecimen may be illuminated with a light intensity sufficient to modifythe depletion layer so that it has a measurement width of no greaterthan about 90%, e.g., 80% or less, of the dark equilibrium value. An ACvoltage signal (which is associated with an aspect of the modifieddepletion layer) induced in the specimen by the illumination may bemeasured.

An alternative embodiment of the invention provides a method ofcharacterizing an aspect of a semiconductor specimen in which a targetarea of a surface of the specimen with is illuminated with a lighthaving a first component and a second component; at least the secondcomponent is intensity modulated. A first AC voltage signal (which isassociated with an aspect of a first near-surface region of the specimenhaving a first depth) induced in the specimen by the light is measured.An intensity of the first component of the light is changed to generatemodified light and the target area of the specimen surface isilluminated with the modified light. A second AC voltage signal inducedin the specimen by the modified light is measured. The second AC voltagesignal is associated with the aspect of a second near-surface region ofthe specimen having a second depth that is different from the firstdepth.

A method of measuring an electrical property of a semiconductor specimenin a further embodiment includes illuminating a target area of a surfaceof the specimen with a first light having a first light profile toestablish a first depletion layer having a first width. An intensity ofat least a portion of the first light is modulated and a first ACvoltage signal induced in the specimen by the first light is measured.The first AC voltage signal is associated with an electrical property ofthe first depletion layer. The target area of the specimen surface maybe illuminated with a second light having a second light profile toestablish a second depletion layer having a second width. The secondwidth differs from the first width. (The first and second lights may be,and in most embodiments will be, generated from the same light source orsources by varying an aspect of the source(s).) An intensity of at leasta portion of the second light is modulated and a second AC voltagesignal induced in the specimen by the second light is measured. Thesecond AC voltage signal is associated with an electrical property ofthe second depletion layer.

Still another embodiment provides an apparatus for determining anelectrical property of a semiconductor specimen. The apparatus generallyincludes a light source, an electrode, and a programmable controller.The light source is configured to deliver light comprising a firstcomponent and a second component; the second component is intensitymodulated. The light source is positioned for juxtaposition with asurface of the semiconductor specimen to illuminate a target area of thesurface. The electrode is disposed between the light source and thesurface of the specimen. The controller is programmed to selectivelyvary an intensity of the first component from a first intensity to asecond intensity; receive a first signal corresponding to a voltagebetween the first electrode and the second electrode when the firstcomponent is at the first intensity; receive a second signalcorresponding to a voltage between the first electrode and the secondelectrode when the first component is at the second intensity; anddetermine an electrical property of a portion of a near-surface regionof the semiconductor specimen by comparing the first and second signals.

For ease of understanding, the following discussion is subdivided intothree areas of emphasis. The first section discusses aspects ofsemiconductor characterization systems in accordance with someembodiments of the invention. The second section outlines semiconductorcharacterization methods in accordance with other embodiments of theinvention. The third section explains a suitable methodology forquantifying a dopant concentration of a portion of the semiconductor.

Semiconductor Characterization Systems

FIG. 4 schematically illustrates a semiconductor characterization system100 in accordance with one embodiment of the invention. The surfacecharacterization system 100 generally includes a light source 110, afirst electrode 140, a second electrode 142, an amplifier 150, and acontroller 160. In FIG. 4, the second electrode 142 is in direct contactwith the back surface 54 of a semiconductor specimen 50 and is directlycoupled to the amplifier 150. In an alternative embodiment, the system100 may instead support the specimen with a conductive specimen holder(e.g., 202 in FIG. 5) and both the specimen holder and the second inputof the amplifier may be connected to ground.

The light source 110 is positioned to direct a beam of light L throughthe first electrode 140 to illuminate a measured surface 52 of thespecimen 50, which is schematically typified as a p-type semiconductorin FIG. 4. The measured surface of the semiconductor specimen 50 carriesa surface charge having the same polarity as the majority carriers,i.e., a positive charge for the p-semiconductor specimen 50. The surfacecharge may be due to a natural phenomenon or induced, e.g., bydepositing a corona charge. If the surface charge is high enough, aninversion layer is formed at the surface 52 and the depletion layer D₀may reach a maximum equilibrium width W_(m0) in the absence of anyillumination by the light beam L.

Generally, the average height of the surface potential and the width ofthe depletion layer D₀ decreases under illumination by the light beam L.If the light beam L has a very low intensity, the change of the surfacepotential and the width of the depletion layer will be negligible, assuggested in U.S. Pat. No. 4,544,887. The AC-SPV becomes a non-linearfunction of the light intensity at higher intensities, though. (See,e.g., Kamieniecki, 1992, in the preceding bibliography.) Thedifficulties associated with this non-linear function have effectivelyprecluded use of AC-SPV measurements from high light intensities toreliably determine electrical properties, e.g., dopant concentration, ofsemiconductors.

In embodiments of the invention, the light beam L has a higherintensity. In particular, the light beam may be intense enough tomaterially reduce the surface potential of the specimen 50 and the widthW_(m1) of the resultant depletion region D₁ from their dark equilibriumvalues W_(m0) and D₀. As explained in more detail below, embodiments ofthe invention characterize at least one aspect of a semiconductor byselectively controlling an aspect of such a high-intensity light. Forexample, by controlling the intensity of a high-intensity light beam L,the width W_(m1) of the depletion region D₁ can be selected tocorrespond to, or even be less than, the thickness of a layer or film ofinterest, such as a thin active film having a thickness of 100 nm orless. In one embodiment, light beam L will yield a depletion layer D₁having a width W_(m1) that is no more than 90%, e.g., 80% or less, ofthe dark equilibrium value of the width W_(m0) of the depletion layerD₀. In one more particular embodiment, the intensity of the light beam Lis sufficient to yield a depletion layer width W_(m1) during measurementthat is no great than about 50% of the dark equilibrium value W_(m0). Itis anticipated that select embodiments of the invention may be able toreduce this percentage to 20% or less, e.g., 10%. If so desired, thecontroller 160 may be coupled to the light source 110 to control theselected aspects, e.g., the intensity, of the light beam L. In someembodiments, the controller 160 may also be used to analyze signals fromthe amplifier 150.

Embodiments of the invention employ a light source 110 configured togenerate a light beam L including a first component 120 and a secondcomponent 130. The first and second components 120 and 130 may begenerated from a single common light emitter (as discussed below inconnection with FIG. 5) or employing two or more independentlycontrollable light emitters (as discussed below in connection with FIG.7, for example). In accordance with embodiments of the inventiondetailed below, selective variance of the first light component 120 canbe used to control the width of the W_(m1) of the depletion layer D₁generating the AC-SPV signal and, hence, the width of the near-surfaceregion of the specimen 50 being characterized with the semiconductorcharacterization system 100.

In one embodiment of the invention, the first light component 120 may beused to reduce the average surface potential and, hence, the widthW_(m1) of the resultant depletion region D₁. The second light component130, in contrast, is intensity modulated and generates the alternatingcurrent in the AC-SPV. As noted previously, the relationship betweenAC-SPV and light intensity is essentially linear at very low lightintensities, but is non-linear at higher intensities. This is caused bythe dependence of the surface photovoltage signal on light intensityvarying not only with changes in doping concentration, but also withchanges in the height of the surface potential barrier, and theresultant width W_(m1) of the depleted region D₁.

For strongly inverted surfaces, the effect of surface traps isnegligible, so the height of the surface potential barrier for a givenlight intensity can be determined empirically using a series ofstandardized calibration wafers having known dopant concentrations, asdescribed below. This empirically determined relationship will allow afairly accurate and precise quantification of the average dopantconcentration within the depletion layer D₁ and the width W_(m1) of thedepletion layer D₁ for a given light intensity. Furthermore, thedepletion layer width W_(m1) can be varied in a predictable manner bychanging the light intensity, permitting incremental analysis of thenear-surface region of a specimen 50, as discussed below. In addition,the higher light intensity will increase the strength of the AC-SPVsignal, achieving a higher signal-to-noise ratio in the output signal ofthe amplifier 150. Alternatively, the light beam L may be focused on asmaller surface area of the specimen surface 52 for an AC-SPV havingabout the same signal-to-noise ratio as conventional low-intensityAC-SPV-based systems. This facilitates characterization of much moreprecisely targeted areas of a specimen 50.

FIG. 5 illustrates a semiconductor characterization system 200 inaccordance with a more specific embodiment of the invention. Aspects ofthe system 200 are analogous to aspects of the more general blockdiagram of FIG. 4. Such analogous elements bear similar referencenumerals in FIGS. 4 and 5, but the reference numerals in FIG. 5 areincremented by 100. Hence, the light system 210, electrode 240,amplifier 250, and controller 260 of FIG. 5 may be analogous to thelight system 110, first electrode 140, amplifier 150, and controller160, respectively, of FIG. 4.

FIG. 5 also illustrates a specimen holder 202 configured to support thesemiconductor specimen 50 with respect to the light source 210. Thisholder 202 may be connected to ground 206 or another common referencevoltage with the ground 252 of the amplifier 250 to measure the voltageacross the specimen 50 to be measured. As shown schematically by acapacitor in FIG. 5, a dielectric support 204 (e.g., an array of pinsdefining an air gap or a dielectric plastic layer, e.g., polyurethane)may be disposed between the specimen 50 and the support 202.

The holder 202 may be driven by a motor 208 adapted to position theholder 202 and the specimen 50 it carries with respect to the lightsource 210. This enables a particular target area of a specimen surface52 to be positioned for analysis using the light beam L. As suggested bythe arrows in FIG. 5, the motor 208 may be adapted to move the holder202 in an X-Y plane.

The light source 210 of FIG. 5 includes a light emitter 222 adapted togenerate light of the desired wavelength and intensity. Light from thelight emitter 222 may be directed toward the specimen 50 via a lightguide, typified in FIG. 5 as a fiber optic cable 224. This fiber opticcable 224 can be coupled at one end to the emitter 222 and at the otherend to a fiber optic collar 212 holding the front end in positionrelative to the electrode 240.

The light emitter 222 may take any of a variety of forms. In oneembodiment, the light emitter 22 emits light, e.g., monochromatic light,at a wavelength that is shorter than a wavelength corresponding to anenergy gap of the target area of the specimen surface 52 illuminated bythe light beam L (e.g., area 52 _(L) in FIG. 10, discussed below). Forexample, the light emitter 222 may comprise a UV diode laser generatinglight with a wavelength between about 365 nm and about 405 nm; such UVdiode lasers are commercially available. In another implementation, thelight emitter 222 may comprise CW diode-pumped solid state lasers havinga wavelength of about 266 nm. Generally, shorter wavelengths arepreferred over longer wavelengths.

The light source 210 may also include a lens 214, which may comprise oneor more lenses. In one embodiment, the lens 214 is configured tocollimate the light beam L so that the dimension of the target area onthe specimen surface 52 illuminated by the light beam L will not varywith distance between the light source 210 and the specimen 50. Inanother embodiment, the lens 214 may be configured to focus the lightbeam L on a smaller spot on the substrate surface 52. The size of thisspot will vary from one application to another, but a size of about1-100 μm should work wall for most applications. One exemplary systemuseful for characterizing an area within the scribe line on a patternedsemiconductor wafer, for example, focuses the beam on an area of about20 μm. The lens 214 may comprise any known lens or system of lensesachieving the desired effect.

For example, the lens 214 may comprise one or more fused silica lensesor a gradient index lens.

An electrode 240 is disposed between the light source 210 and thespecimen surface 50. The electrode 240 is at least generally transparentto the light beam L. In one particular embodiment, the electrode 240comprises a generally circular disk of quartz or fused silica bearing anindium-tin-oxide (ITO) coating on the surface facing the specimen 50 andon the periphery of the disk. A metal ring (not specificallyillustrated) may extend around the periphery in electrical contact withthe ITO coating and be connected to a lead line that communicates withthe amplifier 250.

The electrode 240 is moveable with respect to the specimen holder 202 bya Z-axis motor 216. The motor 216 moves a body 218 to which theelectrode 240, optical cable collar 212, and lens 214 are rigidlyattached to maintain a fixed relationship. The controller 260 maycontrol operation of the motor 216 to maintain spacing between theelectrode 240 and the specimen surface 52 in a desired range, e.g.,10-100 μm, by monitoring a capacitance signal from the electrode 240.The capacitance signal may be generated by coupling an ac voltage source205 to the specimen 50 via the wafer holder 202. As the distance betweenthe electrode 240 and the specimen surface 52 varies, so will thecapacitance. To avoid interference between the capacitance signal andthe AC-SPV, both of which may be measured with the same electrode 240,the ac voltage frequency may be different from the frequency of thesecond light component 130 of the light beam L, e.g., the ac voltage mayhave a frequency of about 70 kHz and the second light component 130 mayhave a frequency of about 30 kHz.

As noted above in the discussion of FIG. 4, adjusting the first lightcomponent 120 can effectively alter the width W_(m1) of the depletionlayer D₁; the second light component 130 is intensity modulated andgenerates the alternating component of the AC-SPV. The graphs in FIGS.6A-B schematically illustrate one manner in which the first and secondlight components 120 and 130 may be varied by the light emitter 222 ofFIG. 5. The waveform A in FIG. 6A is a square wave form, with theintensity shifting between a first intensity I₁ (arbitrarily set at theabscissa) to a second, higher intensity I₂. The amplitude of thiswaveform, i.e., the difference between these two intensities I₁ and I₂,defines the intensity of the second light component 130. The averageintensity I₃ of the illumination may be considered the first lightcomponent 120 in that this average intensity is largely what defines thedifference between the average width W_(m1) of the depleted layer D₁ andthe width W_(m0) of the equilibrium depletion layer D₀.

The wave form B in FIG. 6B is similar to wave form A in that it is asquare wave form and, in one embodiment, has the same amplitude betweenits maximum and minimum intensities I₁ and I₂ (and hence has a secondlight component 130 of the same intensity) as wave form A. However, theactual intensity of the maximum and minimum intensities I₁ and I₂ arehigher in wave form B than in wave form A, so the average intensity I₃,and hence the intensity of the first light component 120, is greater inFIG. 6B than in FIG. 6A. Hence, by selectively altering the modulationof light intensity generated by the light emitter 222, the first lightcomponent 120 of the light beam L may be varied from one measurement tothe next without necessitating any change in the second light component130 of the same light beam.

The controller 260 of the semiconductor characterization system 200 ofFIG. 5 may take the form of one or more computers including one or moreprogrammable processors 266. The processor 266 is adapted to carry outmethods in accordance with further embodiments of the invention (e.g.,the methods discussed under heading C below). For example, thecontroller 160 may be used to control the intensity of the light beam L,including the amplitude of the modulation, to selectively control theintensity of the first and second light components 120 and 130.

The controller 260 may also be adapted to process signals from theamplifier 250 to determine characteristics of a near-surface region ofthe specimen 50. As suggested in block form in FIG. 5, the controller260 generally includes an analog-to-digital converter 262, a digitalsignal processor 264, and the computer processor(s) 266 noted above. Thesignal from the amplifier 250 may be delivered to the analog-to-digitalconverter 262, which outputs a digital signal to the digital signalprocessor 264. The controller 260 may also include a filter (not shown)to filter out noise in the input to the analog-to-digital converter 262.The digital signal processor 264 may be configured to extract the firstharmonic of the signal and separate the first harmonic into twoorthogonal components in a function analogous to that performed by ananalog lock-in amplifier. Such digital signal processors are well knownin the art and need not be detailed here. The signal from the digitalsignal processor 264 may be perceived by the computer 266 and processed,e.g., in accordance with one of the methods outlined below. FIG. 7schematically illustrates a semiconductor characterization system 300 inaccordance with an alternative embodiment of the invention. Someelements of this system 300 may be substantially the same as elements ofthe semiconductor characterization system 200 of FIG. 5 and suchelements bear like reference numerals in FIGS. 5 and 7.

One difference between the semiconductor characterization systems 200and 300 is the nature of the light beam L. The light source 210 of FIG.5 has a single light emitter 222 that generates both the first lightcomponent 120 and the second light component 130. In contrast, the lightsource 310 of FIG. 7 includes a first light element 210 a and a secondlight element 210 b. The first light element 210 a comprises a firstlight emitter 322 and a light guide 324, e.g., a bundle of fused quartzoptical fibers, configured to guide light along a first path toward thespecimen surface 52. Similarly, the second light element 210 b comprisesa second light emitter 332 and a light guide 334, which may besubstantially the same as the light guide 324, is configured to guidelight along a second path toward the specimen surface 52. Both of thelight guides 324 and 334 may be coupled to the fiber optic collar 212.In an embodiment, the first and second light elements 210 a and 210 bare mixed together within the collar 212, e.g., by randomly comminglingthe optical fiber strands of the light guides 324 and 334. In analternative embodiment (not shown), light from each of the lightelements 210 a and 210 b may be guided independently toward the sametarget area of the specimen surface 52, e.g., by being oriented at anangle with respect to one another. This angle is desirably relativelysmall, though, so both light elements 210 a and 210 b impinge on thesurface 52 at an angle as close to perpendicular as reasonablypracticable. In another alternative approach, the light from the firstand second light emitters 222 and 232 may be mixed, e.g., usingpolarization optics, before entering a common light guide, which may besimilar to light guide 224 in FIG. 5.

The first and second light elements 210 a and 210 b both deliver light,e.g., monochromatic light, having a wavelength that is shorter than awavelength corresponding to an energy gap of the target area of thespecimen surface 52 illuminated by the light L. Generally, shorterwavelengths are preferred over longer wavelengths. Wavelengths of thefirst and second light emitters 322 and 332 may be substantially thesame, e.g., both may be lasers or diodes generating light at the samewavelength. In another embodiment, the wavelengths of the first andsecond light emitters 322 and 332 are different from one another. If thewavelength of the first light emitter 322 is longer than the wavelengthof the second light emitter 332, the first light emitter 322 may be lessexpensive and/or more powerful than the shorter-wavelength second lightemitter 332, reducing cost and/or improving performance of thesemiconductor characterization system 300. Devices noted above as beingsuitable for the light emitter 222 of FIG. 5, e.g., UV diode lasers orCW diode-pumped solid state lasers, may also be useful as the lightemitters 322 and 332 of FIG. 7.

Although the first and second light emitters 322 and 332 may be variedtogether, in one exemplary embodiment the first and second lightemitters 322 and 332 are independently controllable. These lightemitters 322 and 332 may be operatively coupled to the controller 260,which may be configured to independently vary the light intensitygenerated by each of the light emitters 322 and 332, e.g., byselectively varying the intensity of the first light emitter 322 fromone measurement to another and modulating the intensity of the secondlight emitter 332, e.g., in a chopped, square wave form adapted togenerate an AC-SPV.

In the embodiment of FIG. 5, the single light emitter 222 was controlledto generate first and second light components 120 and 130 of varyingintensities. In one particular embodiment, the first light element 210 aof FIG. 7 is configured to provide a non-modulated light that does notcontribute to the second component 120 of the light beam L. The secondlight element 210 b may provide an intensity-modulated light similar tothat of the light emitter 222 of FIG. 5. Hence, as explained above inconnection with FIG. 6A, this second light element 210 b will contributeto both the first light component 120 and the second light component130. If so desired, though, the intensity of the first light component120 may be adjusted without altering the second light element 210. Inthe context of FIG. 6, for example, the first light element 210 a mayhave a first intensity (e.g., the first light emitter 322 may be off)when generating waveform A and a second, higher, intensity whengenerating waveform B. The increase in intensity of the first lightelement 210 a is reflected in FIG. 6B by the increase 210 a in the firstlight intensity I₁. Hence, the second light component 130 may begenerated entirely by the second light element 210 b, but both the firstlight element 210 a and the second light element 210 b contribute to thefirst light component 120.

The area illuminated by the first and second light elements 210 a and210 b may be substantially coextensive. In one useful embodiment, thetwo elements 210 a and 210 b may illuminate areas of differentdimension. The area illuminated by the first light element 210 a isdesirably at least as great as the area illuminated by the second lightelement 210 b. In one particular embodiment, the first light element 210a illuminates a surface area that is larger than and encompasses asurface area illuminated by the second light component element 210 b.For example, the first light element 210 a may illuminate substantiallythe entire forward surface 52 of the specimen 50, with the second lightelement 210 b illuminating a more focused region under test.Illuminating a larger area with the first light element 210 a than withthe second light element 210 b can be particularly useful whenattempting to determine the characteristics of a localized target area,e.g., measuring within a scribe line between adjacent integrated circuitdies on a patterned semiconductor wafer.

FIGS. 8-13 illustrate aspects of a further embodiment of the invention.FIG. 8 schematically illustrates just selected components of asemiconductor characterization system 400. In particular, FIG. 8illustrates only the light source 410 and the specimen 50. The balanceof the semiconductor characterization system 400 may directly parallelthe other elements in FIG. 5 (e.g., specimen holder 202, controller 260,etc.)

The light source 410 of FIG. 8, which is shown in cross-section in FIG.9, includes three light components. The first light element 210 a andthe second light element 210 b may be substantially identical to thoseof FIG. 7 and like reference numbers are used in both figures toindicate like elements. The third light element 210 c of light source410 includes a third light emitter 442 and a guide 444, typified as anoptical fiber cable. The first, second, and third light guides 324, 334,and 444, may be joined together in a common fiber optic collar 412. Asbest seen in FIG. 9, the optical fiber strands of the first and secondguides 324 and 334 may be commingled in a central area 320, e.g., acentral circular area, as shown, within the collar 412. The third guide444 encircles the central region 320. Preferably, the fiber opticstrands from the first and second guides 324 and 334 are not commingledwith the fiber optic strands of the third guide 444. This will result ina light beam L, which may be substantially similar to the light beam Lin FIG. 7, generally centered within an annular “guard” beam B. If sodesired, a lens 414 may be disposed between the fiber optic collar 412and the specimen 50. In embodiments of the invention, the lens 414 maybe configured to focus the light beam L and the guard beam B toward asmaller target area of the specimen surface 52. Suitable lenses 414 maybe similar to those noted above as suitable for use as lenses 214, e.g.,one or more fused silica lenses or a gradient index lens.

FIGS. 10 and 11 schematically illustrate an illuminated target area 52_(T) of the specimen surface 52 (FIG. 4). A characterized area 52 _(L),which is illuminated by the light beam L, is generally centered withinand encircled by a guard illumination area 52 _(B), which is illuminatedby the guard beam B. The relative dimensions of the characterized area52 _(L) and the guard illumination area 52 _(B) can be varied within awide range. As noted above in connection with FIG. 5, the areailluminated by the light beam L (i.e., characterized area 52 _(L) inFIG. 10) may be about 1-100 μm, e.g., about 20 μm in diameter. The outerdiameter of the guard illumination area 52 _(B) is desirably at leastabout 40 μm larger than the characterized area 52 _(L), i.e., it mayextend about 20 μm or more radially outwardly from the periphery of thecharacterized area 52 _(L). In selected embodiments of the invention,the guard illumination area 52 _(B) may extend radially outwardly fromthe periphery of the characterization area 52 _(L) about 20-1000 μm.

As shown in FIG. 11, the intensity of the guard beam B (FIG. 8) isdesirably substantially higher than the intensity of the light beam L(FIG. 8) illuminating the characterized area 52 _(L). As discussedabove, the intensity modulation of the second light component 230 willmodulate the intensity of the light beam L. In one embodiment of theinvention, though, the guard beam B may be substantially continuous,i.e., not intensity modulated. In practice, it is not critical to keepthe intensity within the guard area 52 _(B) uniform far from the centralcharacterized area 52 _(L). Maintaining a high, uniform light intensityin the immediate vicinity of the characterized area 52 _(L) isanticipated to be beneficial, though.

FIGS. 12 and 13 schematically illustrate the relative surface potentialsof the characterized area 52 _(L) and the guard illumination area 52_(B). FIG. 12 plots the height of the surface potential barrier as afunction of distance inwardly from the surface 52 of the semiconductorspecimen 50 (FIG. 4). The upper solid curve indicates the limit of theconduction band for the characterized area 52 _(L), whereas the upperdashed curve represents the limit of the conduction band in the guardillumination area 52 _(B). Similarly, the lower solid curve indicatesthe limit of the valance band in the characterized area 52 _(L) and thelower dashed curve indicates the limit of the valance band in the guardillumination area 52 _(B). It is also worth noting that the depletionlayer width W_(L) associated with the characterizing area 52 _(L) iswider than the depletion layer width W_(B) associated with the guardillumination area 52 _(B). This difference reflects the differentintensities of the light beam L and the guard beam B (FIG. 9). FIG. 13schematically illustrates height of the surface potential barrier acrossa width of the surface target area 52 _(T). As in FIG. 12, the solidportion of the curve is associated primarily with the characterizingillumination area 52 _(L) and the dashed part of the curve is associatedwith the guard illumination area 52 _(B). Both curves reflect the highersurface potential barrier in the characterized area 52 _(L) with respectto the guard illumination area 52 _(B). The lower surface potentialbarrier encircling the characterized area 52 _(L) can effectivelyconfine the electrons (in a p-type semiconductor) within thecharacterized area 52 _(L), reducing any spread of the area beingcharacterized that may otherwise occur if the light beam L is usedalone.

Methods for Characterizing Semiconductors

Other embodiments of the invention provide methods for characterizing anaspect of a semiconductor specimen. For ease of understanding, themethods outlined below are discussed with reference to the semiconductorcharacterization systems 100, 200, 300, or 400 of FIGS. 4-13. Themethods are not to be limited to any particular system illustrated inthe drawings or detailed above, though; any apparatus that enablesperformance of a method of the invention may be used instead.

One embodiment of the invention allows determination of the dopantconcentration in a near-surface region of the semiconductor specimen 50.In this embodiment, a semiconductor specimen 50 of interest may beprepared for analysis, if necessary, by forming a thin oxide film on thespecimen surface 50 and thermally reactivating the dopant or dopants(e.g., boron) by thermal treatment. Both of these processes areconventional. Some specimens 50 may already have a surface chargesufficient to induce an inversion layer, but it may be necessary toincrease the sufficient surface charge of other specimens. In oneembodiment, this is done before the specimen 50 is placed in the sampleholder 202 (FIG. 5), e.g., by depositing a corona charge. Alternativelyor in addition to the corona charge, the specimen 50 may be charged bythe semiconductor characterization system 100 during measurement.

When suitably prepared, the specimen 50 may be placed in the specimenholder 202 and positioned with respect to the light source 110 and thefirst electrode 140 to achieve the desired spacing between the specimensurface 52 and the electrode 140 and to position a target area of thespecimen surface 52 for illumination by the light source 110. The lightsource 110 may be activated during positioning of the specimen 50 orjust before a measurement is to be taken. The activated light source 110will illuminate the target area with a light beam L that includes afirst light component 120 and a second light component 130. The AC-SPVgenerated by the light beam L will induce a signal in the electrode 140,which is communicated to the controller 160 via the amplifier 150. Thecontroller 160 may record this signal as a first ac voltage signal. Ifso desired, this signal may be used (in a manner discussed below) todetermine a first doping concentration of a first near-surface region ofthe specimen 50.

As noted before, the width W_(m1) of the depletion layer D₁characterized by a light beam L will vary with the intensity of thelight beam L. In embodiments of the invention, the intensity of thelight beam L can be selected to measure a target depth that iscorrelated to a depth of interest. For example, the specimen 50 may be asemiconductor wafer that has been doped, e.g., by ion implantation,during one stage of production. This doped region will have ananticipated thickness, which may be empirically determined from priorproduction runs, and the target thickness may be correlated to thisanticipated thickness. This correlation may vary depending on the natureof the specimen. For some specimens, the target thickness may be equalto the anticipated thickness. For others, e.g., where a heavily dopedregion underlies a thin film, it may be beneficial to make the targetthickness thinner, e.g., 10% or 20% thinner, when measuring the thinfilm. In other circumstances, it may be beneficial to use a targetthickness that exceeds the thickness of the layer of interest.

An aspect of the intensity of the light beam L may be changed togenerate a modified light beam L′ (not shown). In one embodiment, thiscomprises changing the intensity of the first light component 120. If sodesired, the intensity of the second light component 130 may also bevaried, but the intensity of the second light component 130 in oneuseful embodiment remains substantially constant from one measurement tothe next, at least for a particular target area of a particular specimen50. The modified light beam L′ illuminates the target area of thespecimen surface 52, generating a second AC-SPV, which ultimately may berecorded as a second ac voltage signal by the controller. This secondsignal may be used to determine a second doping concentration in asecond near-surface region of the specimen.

As explained above, the difference in intensity of the first light beamL and the modified light beam L′ will change the height of the surfacepotential barrier of the target area which, in turn, changes the widthW_(m1) of the depletion layer D₁. A first AC-SPV generated by the firstlight beam L is, therefore, characteristic of a near-surface region ofthe specimen 50 having a first width W_(m1) and the AC-SPV generated bythe modified light beam L′ is characteristic of a near-surface region ofthe specimen 50 having a different second width W_(m2). Hence, one ofthe two widths will include an incremental width that is not included inthe other. For example, if the intensity of the first light beam L isless than the intensity of the modified beam L′, the first width W_(m1)will be wider than the second width W_(m2), so the first width W_(m1)will include an incremental width W_(Δ) of the specimen 50 that is notincluded in the second width W_(m2), i.e., W_(m1)−W_(m2)=W_(Δ).

The difference in the first AC-SPV and the second AC-SPV may, therefore,be attributed to the incremental width W_(Δ). In one embodiment, thefirst doping concentration and the second doping concentration may beseparately calculated, as noted above. The difference between the twodoping concentrations may be attributed to the region encompassed in oneAC-SPV measurement and not the other, i.e., W_(Δ), in the precedingexample. By selectively adjusting the light intensity to generate asecond modified light beam L″, a third doping concentration can bedetermined for a depletion layer having a third width W₃.

The same process outlined above may then be used to characterize thedopant concentration of a second incremental width between the thirdwidth W_(m3) and the first or second width W_(m1) or W_(m2). Continuingthis process for a series of successive widths W_(m1), W_(m2), W_(m3),etc. generates a series of dopant concentrations for a series ofdifferent incremental widths that may be used to generate a dopantconcentration profile. FIG. 14 illustrates one such profile, with dopantconcentration (N_(SC)) shown on the ordinate and the location (X informula G below) associated with each incremental width on the abscissa.

System Calibration and Determination of Dopant Concentration

The following discussion sets forth one useful way to correlate measuredAC-SPV signals with a dopant concentration in embodiments of theinvention. Other embodiments may use any other suitable approachinstead. Like the preceding discussion of select methods of theinvention, the following discussion refers to specific systems outlinedabove, but other suitable apparatus may be used instead. Thecalculations outlined below may be performed by a processor of thecontroller 160, e.g., using an ASIC or executable software.

System Calibration

The signal phase of the second light component 120 of the light beam Lis adjusted using the programmed controller 160 at low light intensity,i.e., at an intensity wherein the SPV signal is directly proportional tolight intensity, using a thermally oxidized semiconductor specimen withlow surface state density, i.e., low surface recombination. Thecontroller may adjust this phase until the ratio of the real component(Re) to the imaginary component (Im) reaches a value of −2/π. Therotation angle required to achieve this ratio is stored for use withmeasured specimens.

The intensity of the light beam L may be measured using a calibratedphoto-detector. The light intensity may be changed, e.g., by changingpower delivered to the light source 110 or by using neutral intensityfilters (not shown), until the measured intensity falls within apredetermined acceptable range. This calibration process sets the lightintensity, and thus the generation rate (G) of electron-hole pairs inthe near-surface region, which is tied to the light intensity, at aknown value.

Determining Gain of Amplified AC-SPV Signal

The gain of the signal amplification channel (including couplingcapacitances) may be determined by comparing a measured imaginarycomponent of the amplified AC-SPV signal, ImV_(Fm), to a calculatedimaginary component, ImV_(F), of the signal. A surface charge may bedeveloped on a uniformly doped specimen, e.g., an n-type siliconspecimen, of known doping concentration (e.g., 10¹⁶ cm⁻³) that is coatedwith native oxide. The surface charge may be developed using a coronawire, for example. The charging may continue until the signal from theamplifier 150 reaches saturation, indicating that the specimen surface52 has reached inversion conditions.

The calibrated light source 110 may be activated to illuminate a portionof the specimen surface 52 with the light beam L, which has a knownintensity and electron-hole generation rate G. The dependence ofimaginary component of the signal, ImV_(Fm), on the electron-holegeneration rate G is measured. The normalized ImV_(Fm)/G signalsaturates at low intensities, i.e., when ImV_(Fm) depends linearly onthe light intensity. FIG. 15 plots the ratio of the normalized signaldivided by its low intensity normalized value, given by Formula A

${\frac{{Im}\; V_{Fm}}{{Im}\; V_{Fmo}}\frac{G_{o}}{G}},$where G_(o) denotes the generation rate at which the normalized signalreaches its saturation value ImV_(Fmo).

The calculated imaginary component ImV_(F) of the signal may becalculated based on recombination properties of the surface region. Thecalculation assumes the semiconductor surface is illuminated with ashort wavelength radiation that penetrates to a depth substantiallyshorter than the width of the depletion layer. This illumination is alsoassumed to be 100% square-wave intensity modulated. The imaginary,ImV_(F), and real, ReV_(F), components of the signal are givenrespectively by ImV_(F)=ImV_(Fi)+ImV_(Fd) (Formula B) andReV_(F)=ReV_(Fi)+ReV_(Fd) (Formula C), where

$\quad\begin{matrix}{{{Im}\; V_{Fi}} = \begin{matrix}{{{- \frac{k\; T}{2\pi\; q}}{\int_{- \pi}^{0}{{\ln\left\lbrack {1 - {a_{i}{\exp\left( {{{- \theta}/\omega}\;\tau_{i}} \right)}}} \right\rbrack}\sin\;\theta\;{\mathbb{d}\theta}}}} -} \\{{\frac{kT}{\pi\; q}{\ln\left\lbrack {\frac{G}{v_{i}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack}},}\end{matrix}} \\{{{{Im}\; V_{Fd}} = {{{- \frac{k\; T}{2\pi\; q}}{\int_{0}^{\pi}{{\ln\left\lbrack {1 - {a_{d}{\exp\left( {{{- \theta}/\omega}\;\tau_{d}} \right)}}} \right\rbrack}\sin\;\theta\;{\mathbb{d}\theta}}}}\; - {\frac{1}{\pi}V_{so}}}},} \\{{{{Re}\; V_{Fi}} = {\frac{k\; T}{2\pi\; q}{\int_{- \pi}^{0}{{\ln\left\lbrack {1 - {a_{i}{\exp\left( {{{- \theta}/\omega}\;\tau_{i}} \right)}}} \right\rbrack}\cos\;\theta\;{\mathbb{d}\theta}}}}},\mspace{14mu}{and}} \\{{{Re}\; V_{Fd}} = {\frac{k\; T}{2\pi\; q}{\int_{0}^{\pi}{{\ln\left\lbrack {1 - {a_{d}\exp\;\left( {{- \theta}/{\omega\tau}_{d}} \right)}} \right\rbrack}\cos\;\theta\;{{\mathbb{d}\theta}.}}}}}\end{matrix}$

Coefficients a_(i) and a_(d) are determined as follows:

$\quad\begin{matrix}{{a_{i} = {1 + \frac{\begin{matrix}{{{\exp\left( {{- \pi}/{\varpi\tau}_{d}} \right)}\left\lbrack {1 - {\exp\left( {{{- \pi}/\omega}\;\tau_{i}} \right)}} \right\rbrack} +} \\{\left\lbrack {1 + {\frac{G}{v_{t\;}N_{sc}}{\exp\left( {q\;{V_{mo}/k}\; T} \right)}}} \right\rbrack\left\lbrack {1 - {\exp\left( {{{- \pi}/\varpi}\;\tau_{d}} \right)}} \right\rbrack}\end{matrix}}{{\exp\left( {{{- \pi}/{\omega\tau}_{d}} - {\pi/{\omega\tau}_{i}}} \right)} - 1}}},} \\{a_{d} = {1 - \frac{\begin{matrix}{{{\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}\left\lbrack {1 - {\exp\left( {\pi/{\omega\tau}_{i}} \right)}} \right\rbrack} -} \\{\left\lbrack {\frac{G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {1 - {\exp\left( {{{- \pi}/\varpi}\;\tau_{d}} \right)}} \right\rbrack}\end{matrix}}{\left\lbrack {\frac{G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {{\exp\left( {{{- \pi}/\omega}\;\tau_{d}} \right)} - {\exp\left( {\pi/{\omega\tau}_{i}} \right)}} \right\rbrack}}}\end{matrix}$

In turn, τ_(i), and τ_(d) are determined by the following formulas:

$\quad\begin{matrix}{{\tau_{i} = \left\{ {{\frac{q}{k\; T}\left\lbrack {{v_{t}N_{sc}{\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} + G} \right\rbrack}\sqrt{2q\;{V_{m}/ɛ_{s}}N_{sc}}} \right\}^{- 1}},} \\{{\tau_{d} = \left\lbrack {\frac{q}{k\; T}v_{t}N_{sc}{\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}\sqrt{2q\;{V_{m}/ɛ_{s}}N_{sc}}} \right\rbrack^{- 1}},}\end{matrix}$where q is a magnitude of an elementary charge, kT is a thermal energyat room temperature, N_(sc) is net density of the dopants in the spacecharge region (depletion layer), ∈_(s) is permittivity in semiconductor,and v_(t) is a surface recombination velocity. Magnitudes of theimaginary and real component only weakly depend on v_(t) under inversionconditions under illumination, so in the most cases v_(t)=10³ cm/sec canbe used. V_(mo) is the height of surface potential barrier at the onsetof strong inversion in thermal equilibrium and is given by:

${V_{mo} = {2\frac{k\; T}{q}{\ln\left( \frac{N_{sc}}{n_{i}} \right)}}},$where n_(i) is an intrinsic carrier concentration. V_(m) is the heightof the surface potential barrier under illumination.

The gain of the system is determined in two steps. First, the dependenceof V_(m) on the generation rate for the calibration specimen isdetermined by comparing the normalized measured imaginary componentImV_(F) and normalized calculated imaginary component ImV_(Fo), eachdivided by their low intensity saturation values. The normalizedmeasured imaginary component is given by Formula A, above. Thenormalized calculated imaginary component is given by Formula D:

$\frac{{Im}\;{V_{F}\left( {G,V_{m}} \right)}}{{Im}\;{V_{Fo}\left( {G_{o},V_{mo}} \right)}}\frac{G_{o}}{G}$where ImV_(F) is given by Formula B, above, with doping concentration ofthe measured specimen (10¹⁶ cm⁻³ for the specimen plotted in FIG. 15),and ImV_(Fo) is given by Formula B with the same doping concentrationand the height of the surface potential barrier given by its equilibriumvalue V_(mo). Varying V_(m) for the calibration specimen will indicatethe correlation between the measured and calculated ratios. This, inturn, permits determination of the dependence of V_(m) on the generationrate. The result for the specimen of FIG. 15 is shown in FIG. 16.Knowing the intensity dependence of V_(m), the gain of the signalamplification channel is determined by dividing the measured imaginarycomponent by the calculated imaginary component, i.e.,Gain=ImV _(Fm)(G)/ImV _(F)(G,V _(m)).Determination of Doping Concentration

Approach 1: In one embodiment, both real and imaginary components of thesignal are measured and compared with Formulas B and C, both above.Imaginary and real components depend on three parameters—carriergeneration rate, G, doping concentration, N_(sc), and the height of thesurface potential barrier, V_(m). G for a given light intensity is knownfrom the calibration of the light source 110, so comparison of theimaginary and real components permits the two remaining parameters,N_(sc) and V_(m), to be determined. In turn, knowledge of theseparameters allows the width W_(mn) of a particular depletion layer D_(n)using Formula E:

$W_{mn} = {\sqrt{\frac{2ɛ_{s}V_{m}}{q\; N_{sc}}}.}$

By determining N_(sc) and W_(m) at different generation rates, G′ andG″, (and, hence different depletion layer widths W_(mn) and W_(mn+1))the local doping concentration can be calculated using Formula F:

${{N_{sc}(X)} = \frac{{N_{sc}^{\prime}W_{mn}} - {N_{sc}^{''}W_{{mn} + 1}}}{W_{mn} - W_{{mn} + 1}}},$where the doping concentration N_(sc)(X) corresponds to the dopantconcentration at a depth X calculated using Formula G:X=½(W_(mn)+W_(mn+1)).

This Approach 1 requires precise control of the system phase shift andthe measurement area boundary conditions. A light source such as thelight source 410 of FIG. 8 may enhance control of the boundaryconditions of the area being measured (52 _(L) in FIG. 10). Even so,Approach 1 could introduce unacceptable error into the resulting valuesof the dopant concentrations N_(sc).

Approach 2: An alternative embodiment empirically determines, duringsystem calibration, the dependence of V_(m) on G and N_(sc) for aspecific set of measurement conditions. Subsequently, the dopantconcentration N_(sc) of an unknown specimen may be determined bycomparing measured and calculated values of imaginary components atselected G values. Since imaginary components depend on G, N_(sc), andV_(m), knowledge of G and V_(m) allows N_(sc) and its dependence on thedistance from the surface to be determined using Formulas E and F,above.

One embodiment empirically determines the dependence of the height ofthe surface potential barrier, V_(m), and generation rate, G, on thedoping concentration, N_(sc), employs a number of n-type calibrationspecimens of known, uniform doping concentration. These specimens shouldcover the entire N_(sc) range of interest. Comparison of therelationship between the measured signal and the generation rate G fortwo of such calibration specimens is shown in FIG. 17. The dependence ofthe height of the surface potential barrier, V_(m), on generation rate,G, may be established by matching the measured imaginary component withthe imaginary component calculated using Formulas B and C above. A plotof this dependence is shown in FIG. 18.

In one embodiment, measurements for all specimens are performed at thesame generation rates, G. From the set of measurements on calibrationspecimens of known and uniform doping concentration, dependence of V_(m)on N_(sc) is established for selected values of G. When measuring anunknown specimen, an approximate initial dopant concentration N_(sc)_(—) _(ini) and corresponding initial surface potential barrier heightV_(m) _(—) _(ini) are used to match a calculated imaginary component tothe measured imaginary component, divided by the system gain. If adifference between these values exceeds an acceptable maximum, thecalculation is repeated with the new N_(sc) and corresponding V_(m).This procedure is repeated in an iterative fashion until the differencebetween the calculated imaginary component and the measured imaginarycomponent divided by the system gain is no greater than the acceptablemaximum.

Relying on a constant generation rate may require the electronicdetection system (which comprises the amplifier 150 and controller 160)to have a large dynamic range. To reduce the dynamic range required,other approaches can be used. For example, the measured signal may bemaintained substantially constant by adjusting the light intensity fromthe light source 110. The generation rate, G, for a range of lightintensities may be empirically determined by measuring the lightintensity as a function of power delivered to the light source 110 (oranother factor used to vary intensity, e.g., the arrangement of neutraldensity filters) during calibration of the system 100. The height of thesurface potential barrier, V_(m), may be determined as outlined above,allowing calculation of the doping concentration, N_(sc).

The preceding formulas are useful in connection with embodiments of theinvention employing a light source with a single light emitter, e.g.,the light source 210 of FIG. 5, which employs a single light emitter 222that is 100% modulated, as shown in FIG. 6A. As noted above inconnection with FIGS. 7, 8, and 9, other embodiments of the inventionmay employ two independently controllable light emitters 322 and 332. Insuch an embodiment, the generation rate G attributable to the modulatedsecond light component 130 may be employed in the preceding equations.The specimen may also be illuminated with the non-modulated first lightemitter 322. Adding the additional generation rate, G_(b), attributableto this first light emitter 322, the equations for the imaginarycomponent take the form:

$\quad\begin{matrix}{{{Im}\; V_{Fi}} = {- \begin{matrix}{{\frac{k\; T}{2\pi\; q}{\int_{- \pi}^{0}{{\ln\left\lbrack {1 - {a_{i}{\exp\left( {{- \theta}/{\omega\tau}_{i}} \right)}}} \right\rbrack}\sin\;\theta{\mathbb{d}\theta}}}} -} \\{{\frac{k\; T}{\pi\; q}{\ln\left\lbrack {\frac{G_{b} + G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack}},}\end{matrix}}} \\{{{Im}\; V_{Fd}} = {- \begin{matrix}{{{\frac{k\; T}{2\pi\; q}{\int_{0}^{\pi}{{\ln\left\lbrack {1 - {a_{d\;}{\exp\left( {{- \theta}/{\omega\tau}_{d}} \right)}}} \right\rbrack}\sin\;\theta{\mathbb{d}\theta}}}} +}\ } \\{\frac{k\; T}{\pi\; q}{{\ln\left\lbrack {\frac{G_{b}}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack}.}}\end{matrix}}}\end{matrix}$

The form of equations for the real component remains unchanged fromthose set forth above. Accordingly, the equations for determining thecoefficients a_(i), a_(d), τ_(i), and τ_(d) in the imaginary and realcomponent formulas above become:

$\quad\begin{matrix}{a_{i} = {1 + \frac{\begin{matrix}{{\left\lbrack {\frac{G_{b}}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack{{\exp\left( {{{- \pi}/\varpi}\;\tau_{d}} \right)}\left\lbrack {1 - {\exp\left( {{{- \pi}/\omega}\;\tau_{i}} \right)}} \right\rbrack}} +} \\{\left\lbrack {\frac{G_{b} + G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {1 - {\exp\left( {{- \pi}/{\omega\tau}_{d}} \right)}} \right\rbrack}\end{matrix}}{\left\lbrack {\frac{G_{b}}{{v_{t}N_{sc}}\;} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {{\exp\left( {{{- \pi}/{\omega\tau}_{d}} - {\pi/{\omega\tau}_{i}}} \right)} - 1} \right\rbrack}}} \\{{a_{d} = {1 - \frac{\begin{matrix}{{\left\lbrack {\frac{G_{b}}{v_{t\;}N_{{sc}\;}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {1 - {\exp\left( {{- \pi}/{\omega\tau}_{i}} \right)}} \right\rbrack} -} \\{\left\lbrack {\frac{G_{b} + G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {1 - {\exp\left( {{- \pi}/{\omega\tau}_{d}} \right)}} \right\rbrack}\end{matrix}}{\left\lbrack {\frac{G_{b} + G}{v_{t}N_{sc}} + {\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} \right\rbrack\left\lbrack {{\exp\;\left( {{{- \pi}/\omega}\;\tau_{d}} \right)} - {\exp\;\left( {\pi/{\omega\tau}_{i}} \right)}} \right\rbrack}}},} \\{{\tau_{i} = \left\{ {{\frac{q}{k\; T}\left\lbrack {{v_{t}N_{sc}{\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} + G_{b} + G} \right\rbrack}\sqrt{2\; q\;{V_{m}/ɛ_{s}}N_{sc}}} \right\}^{- 1}},\mspace{14mu}{and}} \\{\tau_{d} = {\left\{ {{\frac{q}{k\; T}\left\lbrack {{v_{t}N_{sc}{\exp\left( {{- q}\;{V_{mo}/k}\; T} \right)}} + G_{b}} \right\rbrack}\sqrt{2\; q\;{V_{m}/ɛ_{s}}N_{sc}}} \right\}^{- 1}.}}\end{matrix}$

The gain of the signal amplification channel may be measured usingessentially the same process detailed above by turning off the firstlight emitter 222, which will set G_(b)=0. The remaining measurementsand the calibration procedure may be performed, e.g., at constant G_(b)and variable G, or at constant G and variable G_(b).

CONCLUSION

Embodiments of the invention, therefore, permit characterization of verythin near-surface regions of semiconductors using a non-contact approachthat will not damage the measured specimen. For example, embodiments ofthe invention may be able to reliably and reproducibly measure dopantconcentrations or other electrical characteristics in near-surfaceregions having a width of 600 nm or less, e.g., no more than 300 nm, fora p-type semiconductor having a dopant concentration of about 10¹⁵ cm³,for example. In select embodiments of the invention, it is anticipatedthat the characterized depth of a p-type semiconductor having a dopantconcentration of about 10¹⁵ cm⁻³ may be about 100 nm or less. Althoughresults may vary somewhat depending on the structural and materialcharacteristics of a specimen, conventional SPV-based characterizationof semiconductors typically cannot reliably measure characteristics oflayers having a thickness more than 1-10% below the maximum equilibriumdepletion layer is width D₀, which will be about 1 μm for a p-typesemiconductor with a dopant concentration of about 10¹⁵ cm⁻³. C-Vmeasurement techniques are typically limited to layers that are at leastabout 500 nm thick for the same p-type specimen with a 10¹⁵ cm⁻³ dopantconcentration.

Further embodiments of the invention are capable of generating a dopantconcentration profile using a non-destructive, non-contact approach.This can facilitate detailed analysis of semiconductor workpieces at oneor more stages in a manufacturing operation, e.g., by measuringsemiconductor wafers between two or more steps in the manufacture ofintegrated circuits or other surface structures. The resultant analysiswill help identify defective products at an early stage in amanufacturing process, saving the sometimes-considerable expense ofcompleting the remaining steps to produce an ultimately defectiveproduct. Neither conventional, low-intensity SPV nor CV approaches offerthe ability to determine a dopant concentration profile of thin films,e.g., layers less than 300 nm thick at a dopant concentration of about10¹⁵ cm⁻³, analogous to that achieved with embodiments of the invention.

The above-detailed embodiments and examples are intended to beillustrative, not exhaustive, and those skilled in the art willrecognize that various equivalent modifications are possible within thescope of the invention. For example, whereas steps are presented in agiven order, alternative embodiments may perform steps in a differentorder. The various embodiments described herein can be combined toprovide further embodiments. Each of the identified patents and each ofthe identified non-patent references (either in the Bibliography orelsewhere) are incorporated herein, in their entirety, by reference.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification unless the preceding description explicitly definessuch terms. The inventors reserve the right to add additional claimsafter filing the application to pursue additional claim forms for otheraspects of the invention.

1. A method of characterizing an aspect of a semiconductor specimen,comprising: selecting a target depth correlated to a depth of interestfor the specimen; selecting a light intensity of a light that has afirst component and an intensity modulated second component, the lightintensity being selected to characterize a target region of the specimenhaving a thickness no greater than the target depth; adjusting the lightto the selected light intensity by varying at least the first componentof the light; illuminating a target area of a surface of the specimenwith the adjusted light; and measuring an AC voltage signal induced inthe specimen by the light, the AC voltage signal being associated withan aspect of the target region of the specimen.
 2. The method of claim 1further comprising determining a doping concentration in the targetregion of the specimen using the AC voltage signal.
 3. The method ofclaim 1 wherein the target region comprises a near-surface region of thespecimen.
 4. The method of claim 1 wherein the semiconductor has adopant concentration no greater than 10¹⁵ cm⁻³ and the target depth isno greater than about 600 nm.
 5. The method of claim 1 wherein thesemiconductor has a dopant concentration no greater than 10¹⁵ cm⁻³ andthe target depth is no greater than about 300 nm.
 6. The method of claim1 wherein the semiconductor has a dopant concentration no greater than10¹⁵ cm⁻³ and the target depth is no greater than about 100 nm.
 7. Themethod of claim 1 wherein the first and second components of the lightare generated by a common light emitter.
 8. The method of claim 7 thefirst component is varied by increasing an average intensity of lightfrom the common light emitter.
 9. The method of claim 7 the firstcomponent is varied by increasing an average intensity of light from thecommon light emitter substantially without changing amplitude of theintensity modulation of the second component.
 10. The method of claim 1wherein the first and second components of the light are generated by atleast two independently controllable light emitters including a firstlight emitter that is not intensity modulated and a second light emitterthat is intensity modulated.
 11. The method of claim 10 whereinadjusting the light to the selected light intensity comprises varyingintensity of light from the first light emitter independently ofintensity of light from the second light emitter.